An Investigation of the Hazard Associated with the Alluvial Fans on the Kaikoura Coast, South Island, New Zealand

Abstract:

Three large alluvial fans on the northeast coast of the South Island between Kekerengu and the Clarence River mouth present a potential hazard to both State Highway 1 and the South Island Main Trunk Rail Line between Picton and Christchurch, which pass over top of them, and to their users. Climate changes are predicted in future for New
Zealand, and may lead to longer warm dry weather periods with heavier rainfall occurrences for the east coast of the South Island. Locations of erosion, the streams' ability to transport sediment and the frequency and magnitude of hazardous fluvial sediment events associated with these alluvial fans could potentially alter due to these
predicted future climatic changes, altering the potential hazard to the state highway and the railway line. The underlying purpose of this thesis is to understand the hazard posed by sediment events onto these three alluvial fans and how this may alter in relation to climate change.
Longitudinal profiles of the stream channels have been surveyed to determine if they are altering from artificial changes to base level and to establish if sediment is dominantly originating from the catchment walls or the stream channel itself. The potential for transporting sediment under varying return periods has been determined through the calculation of stream power and bed shear stress variables. Through a morphological
interpretation of the fans, sedimentation rates have been estimated on both a geological scale and an event scale. The spatially distributed sediment transport model CAESAR was run to determine how the temporal frequency of hazardous events onto the fan could alter in relation to climatic changes. The absence of any knickpoints located within the stream channel profiles implies that the redistribution of sediment through the stream channel is in a steady state and that the catchment and gully walls are the dominant contributor of sediment. Flow events equal to 10- and 23- year return period events provide high amounts of available energy which can be used for sediment entrainment and transport, and ranges between 625.5 W/m/s to 4176.4 W/m/s and 789.7 W/m/s to 5366.2 W/m/s respectively. Values of unit
stream power indicate that a constant amount of potential energy is available per unit area of the stream channel along its length. The comparison of the boundary shear stress and the critical shear stress indicate that all clasts within the stream channel can be entrained by 10- and 23- year return period flow events. Annual geological sedimentation rates for Fan 2 are 181.1 m3/yr, but it is recognised that individual sediment events may have a higher magnitude, with at least 1158.5 m3 sediment deposited on Fan 2 by two events in 2008 collectively. The model runs
suggest that under an altered climatic regime sediment discharge events will generally occur less frequently but will be of a larger magnitude when they do occur. The supply of sediment is unlikely to be exhausted in the near geological future. Under high stream flow discharge events sediment stored in the stream channel is readily able to be entrained and transported, with potentially greater calibre clasts and/or a greater volume of sediment potentially able to be transported as rainfall events become less frequent but more intense. Further, under an altered climatic regime the potential hazard presented to State Highway 1 and the South Island Main Trunk Rail Line and their users is likely to
remain unchanged or increase.